The present invention generally relates to flat-panel display devices and, more particularly, to fabrication of a so-called plasma display panel.
Plasma display panels (PDP) are used in various information display apparatuses, including a wide-screen TV set, as a thin information display device having a very large display area.
FIG. 1 shows the construction of a conventional PDP 10 of the so-called AC type.
Referring to FIG. 1, the PDP 10 includes a rear glass substrate 11 and a front glass substrate 15, wherein the rear glass substrate 11 carries thereon a number of address electrodes 12 each formed of a metal such as Cr, such that each of the address electrodes 12 extends in a column direction on the substrate 11. The glass substrate 1 further carries thereon a dielectric layer 13 of a low melting glass such that the dielectric layer 13 covers the foregoing address electrodes 12. The dielectric layer 13 in turn carries thereon a number of rib patterns 14 each formed of a low melting glass and extending in the column direction, wherein a pair of the rib patterns 14 are disposed at both, i.e., the opposite, lateral sides of an address electrode 12. Further, a fluorescent substance corresponding to one of the three primary colors red (R), green (G) and blue (B) is provided between a pair of the rib patterns 14 in association with the corresponding address electrode 12.
The front glass substrate 15 carries thereon a number of mutually parallel display electrodes 16 of a transparent conductive material such as ITO (In.sub.2 O.sub.3.SnO.sub.2), such that each of the display electrodes 16 extends in a row direction on the glass substrate 15. In the present description, the row direction is defined as being perpendicular to the foregoing column direction.
Each of the display electrodes 16 carries thereon a bus electrode 17 of a metal such as Cr, such that the bus electrode 17 extends on the display electrode in the row direction with a reduced width as compared with the display electrode 16. Further, the glass substrate 15 is covered by a dielectric layer 18 of a low melting glass such that the dielectric layer 18 covers the display electrodes 16 and the bus electrode 17 thereon. Further, a protective film 19 of MgO is provided on the dielectric layer 18.
The glass substrates 11 and 15 are assembled as indicated in FIG. 1 such that the rib patterns 14 on the glass substrate 11 face the protective film 19 on the glass substrate 15. Further, the gap between the substrate 11 and the substrate 15 is filled by a gas such as He, Ne or Xe.
In operation, a drive voltage signal is applied between a selected address electrode 12 and a selected display electrode 17, and a gas plasma is formed in correspondence to the intersection of the foregoing selected electrodes. The gas plasma thus formed causes an optical excitation in the fluorescent substance on the substrate 11 covering the region corresponding to the foregoing intersection.
When fabricating the PDP 10, it is necessary to form the rib patterns 14 on the transparent glass substrate 11 after formation of the address electrodes 12 by conducting a photolithographic process. Similarly, it is necessary to form the bus electrodes 17 on the display electrodes 16 after the formation of the display electrodes 16. When a pattern is to be formed on another pattern by a photolithographic process, it is necessary to carry out an alignment process such that the upper pattern is aligned properly with the lower pattern. In relation to this, it should be noted that the rib patterns 14, formed of a low melting glass, have a whitish, opaque structure.
FIG. 2 shows the construction of an exposure system 20 used in the foregoing photolithographic patterning process when fabricating the PDP apparatus 10 of FIG. 1.
Referring to FIG. 2, the exposure system 20, which includes an Ar laser 21 and an acoustic-optical modulator (AOM) 22 modulating a laser beam 21A emitted by the Ar laser 21, wherein the laser beam 21A thus modulated by the AOM 22 is deflected by a rotary polygonal mirror 23 and scans over the surface of a substrate 27 held on a stage 26, after passing through a lens 24 and a condenser lens 25 consecutively. The substrate 27 may either be the substrate 11 or substrate 15.
FIG. 3 shows the construction of an alignment apparatus used in the exposure system 20 for aligning an upper pattern to a lower pattern.
Referring to FIG. 3, the alignment of the upper pattern to the lower pattern is achieved in the apparatus of FIG. 3 by using an alignment mark as usual in a photolithographic process, and the apparatus of FIG. 3 includes CCD cameras 28A and 28B as well as an image processor 29 cooperating with the cameras 28A and 28B for detection of the alignment mark. In the construction of FIG. 3, it should be noted that the LEDs 26B in respective depressions 26A produce the output optical beams with a wavelength different from the wavelength of the exposure laser beam 21A. Thereby, the output optical beam of the LED 26A or 26B causes no substantial exposure of the photoresist 14A (see FIG. 4B). For example, the LED 26A or 26B may produce a red color beam.
FIGS. 4A and 4B show the cross sectional structure of alignment marks 15A and 11A provided respectively on the substrate 15 and the substrate 11 for the foregoing alignment.
Referring to FIG. 4A, the alignment mark 15A includes an ITO pattern 16A provided on the substrate 15 at a predetermined location, wherein the ITO pattern 16A is covered by a Cr electrode layer forming the bus electrode 17. Thus, the Cr layer is designated by the same reference numeral 17 in FIG. 4A.
In the construction of FIG. 3, the CCD cameras 28A and 28B are used to detect a projection 17A formed on the surface of the Cr layer 17 in correspondence to the ITO pattern 16A as the alignment mark 15A. It should be noted that the Cr layer 17 has actually a Cr/Cu/Cr structure in which a pair of Cr layers sandwich a Cu layer and exhibits a very high reflectance. In FIG. 4A, it should be noted that a resist film 17B is provided on the Cr layer 17.
In the alignment mark 11A of FIG. 4B, on the other hand, an alignment mark pattern 12A having the same Cr/Cu/Cr structure is provided in correspondence to a marginal region of the glass substrate 11 where no rib pattern 14 is formed, simultaneously to the address electrodes 12 that have the same Cr/Cu/Cr structure. In FIG. 4B, it should be noted that the alignment mark pattern 12A is embedded in the thick dielectric layer 13 typically having a thickness of about 10 .mu.m, and thus, the dielectric layer 13 has a substantially flat top surface also in the part covering the alignment mark pattern 12A. Thereby, the detection of the alignment mark pattern 12A by the CCD camera 28A or 28B is not possible as long as the camera 28A or 28B detects a reflection from the surface of the dielectric layer 13. In the construction of FIG. 4B, it should be noted that the dielectric layer 13 carries thereon a resist film 14A such that the resist film 14A covers the rib pattern 14.
Thus, in order achieve a proper alignment detection also for the alignment mark 11A of FIG. 4B, the alignment apparatus of FIG. 3 has a construction in which the stage 26 is formed with a depression 26A in correspondence to the alignment mark pattern 12A for accommodating an alignment optical source 26B, which may be an LED. When the substrate 11 is in a proper position on the stage 26, the alignment mark pattern 12A interrupts the output optical beam of the LED 26B, and the detection of the alignment becomes possible by detecting the state of the output optical beam of the LED 26B by the corresponding on of the CCD camera 28A and 28B. In other words, the alignment detection using the foregoing mark pattern 12A is achieved by the detection of a transmission optical beam, and thus, the mark pattern 12A is called transmission-type alignment mark. In FIG. 3, the LED 26B produces the output optical beam with a wavelength different from the wavelength of about 488 nm of the exposure optical beam 21A. For example, the LED 26B produces a red optical beam.
It should be noted that the alignment apparatus of FIG. 3 is actually the apparatus used for exposing printed circuit boards. In the case of exposing an opaque printed circuit board, a through hole 27A is provided as an alignment mark and the alignment detection is achieved by detecting the output optical beams of the LED 26B by the CCD cameras 28A and 28B.
FIG. 5 shows the process of forming the alignment mark pattern 16A on the substrate by using the exposure apparatus of FIGS. 2 and 3.
Referring to FIG. 5, the glass substrate 15 carrying thereon an ITO film from which the alignment mark pattern 16A is formed and a resist film for pattrning the ITO film, is placed on the stage 26 and the exposure laser beam 21A is applied according to the desired pattern of the alignment mark pattern 16A. The exposure laser beam 21A thus applied passes through the transparent ITO film as well through as the glass substrate 15 underneath and reaches the LED 26B provided on the depression 26A. Thereby, the exposure laser beam 21A is reflected by the surface of the LED 26B and causes an unwanted exposure of the resist film provided on the ITO film. As a result of such an excessive exposure of the resist film, the size of the alignment pattern 16A is modified unwantedly. Further, there may be an anomaly in the display electrode 16 formed simultaneously to the alignment mark pattern 16A from the ITO film.
Further, a similar problem may occur also when the stage 26 is formed with a depression 26C for accepting a robot hand which is used for transporting the substrate.
Referring to FIG. 6, it should be noted that the depressions 26C extend and reach the area of the substrate 15 held on the stage 26 on which the transparent electrode patterns are formed. Thus, the exposure laser beam 21A also causes, when reflected by the depressions 26C, an unwanted extraneous exposure in the resist pattern that is to be formed on the ITO film on the substrate 15 in correspondence to the display electrode 16. Thereby, the pattern of the display electrode 16 may also include a pattern anomaly.
In order to avoid the foregoing problem, it has been practiced to use a completely flat stage for the exposure of the substrate 15, and the alignment apparatus of FIG. 3 is used only for the exposure of the substrate 11. Even in such a case, however, the problem explained with reference to FIG. 6 cannot be avoided.